ILLUSIONS are anomalies that can reveal clues about the mysterious workings of the brain to neuroscientists in much the same way as the fictional Sherlock Holmes can solve a crime puzzle by homing in on a single out-of-the-ordinary fact. Think of the phrase “the dog that did not bark” (in Sir Arthur Conan Doyle's short story “Silver Blaze”) or of the missing dumbbell (in Conan Doyle's Valley of Fear).

Perhaps the most famous examples of such visual tricks are the geometric optical illusions. In the Ponzo illusion (a), first demonstrated by Italian psychologist Mario Ponzo in 1913, one horizontal line looks shorter than the other one, although they are identical. In the Mueller-Lyer illusion (b, on opposite page), created by German psychiatrist Franz Mueller-Lyer in 1889, the line bounded by the diverging arrowheads looks shorter than the one with converging arrowheads—although they, too, are identical.